Continuous solution polymerization processes generally involve the addition of a catalyst to a monomer and solvent mixture. For example, PCT Publication WO 94/00500 describes a solution polymerization process using a metallocene catalyst in continuous stirred tank reactors, which may be parallel or arranged in series, to make a variety of polymer products. Upon reaction of the catalyst and monomers, the formed polymer is dissolved in the polymerization medium or solvent, often along with catalyst and unreacted monomer. Often the solution exiting the polymerization reactor has a relatively low polymer concentration, such as from about 3 wt % to 30 wt %. The product mixture is then passed to polymer concentration and finishing stages to separate the solvent and unreacted monomer from the mixture such that the desired polymer can be recovered in a usable form. The separated solvent and monomer can then later be recycled back to the reactor for re-use.
A polymer solution can exhibit the Lower Critical Solution Temperature (LCST) phenomenon whereby the polymer solution separates into a polymer-rich liquid phase and a polymer-lean liquid phase above a certain temperature. Typically, this separation method involves heating the polymer solution under high pressure, followed by reducing the pressure to a point where two phases (polymer-rich phase and polymer-lean phase) are formed. Of the two phases that are formed, the polymer-lean phase is rich in solvent and contains most of the unreacted monomer and contains very little polymer, whereas the rich phase is polymer rich. The denser polymer-rich phase settles to the bottom of the vessel where it is pressure fed to downstream equipment where the remaining solvent is removed. The solvent-rich phase (polymer-lean phase) overflows out the top of the separation vessel where it is cooled and recycled back to the polymerization reactor for re-use.
If the LCST separation is not carried out at proper operating conditions, there can be incomplete separation of the polymer from the polymer-lean phase. This can lead to polymer product being carried overhead in the lean phase, where it can plate out and detrimentally foul equipment in the recycle solvent stream. Therefore, there remains a need for methods to optimize the recovery of polymer from a solution polymerization process that can minimize or reduce the potential for recycle solvent fouling.
Additional background references include U.S. Pat. Nos. 3,553,156; 3,726,843; 5,264,536; 6,204,344; 6,881,800; 7,163,989; 7,650,930; and 8,916,659; U.S. Patent Application Publication No. 2012/0088893; PCT Publications WO 2011/008955; WO 2013/134041; WO 2013/137962; WO 2013/169357; and Michelsen, M., “A Simple Method for Calculation of Approximate Phase Boundaries”, Fluid Phase Equilibria, 98: 1-11 (1994).